An image sensor converts an optical image into an electrical signal. Image sensors may be classified into complementary metal-oxide-silicon (CMOS) image sensors and charge coupled device (CCD) image sensors. The CCD image sensor may have better photosensitivity and noise characteristics compared with the CMOS image sensor, but may be difficult to fabricate in relatively large scale integration and has higher power consumption than CMOS. In contrast, the CMOS image sensor may have a simpler manufacturing process, leading to higher scale integration, and lower power consumption, compared with CCD image sensors.
Technology for manufacturing the CMOS image sensors has improved, CMOS characteristics have improved, and thus research into CMOS image sensors is ongoing. A pixel of the CMOS image sensor includes photodiodes for receiving light and CMOS components for controlling image signals received from the photodiodes. In the photodiodes, pairs of electrons and holes are generated according to the wavelength and intensity of light of red, green and blue input through color filters and an output signal varies depending on the amount of generated electrons, thereby sensing an image.
A CMOS image sensor may include a pixel region, in which photodiodes may be formed, and a peripheral circuit region for detecting signals generated by the pixel region. The peripheral circuit region may surround the pixel region.
To increase photosensitivity in the CMOS image sensor, a depletion region in a photodiode may be made large. As shown in FIG. 1, an N-type dopant 1 may be ion-implanted at a high energy into a silicon epi layer 10 including a photodiode to deeply form a low-concentration N-type doped region 13.
However, the N-type dopant 1 is implanted at high energy and can sufficiently pass through a gate pattern 12. A gate region cannot be perfectly protected when the N-type doped region 13 is deeply implanted to make the region larger. Thus, the N-type doped region 13 partially overlaps the gate pattern 12 to cause a channeling phenomenon.
The N-type doped region 13 may be made into a source region of a transistor. However, the overlap portion may cause large leakage currents when the CMOS image sensor operates, and a threshold voltage of a transistor located between pixels may not be uniform.